1. Field of the Invention
The present invention relates to an image forming apparatus such as a printer and a copy machine, an image forming method, and a program, and particularly, to an image forming apparatus that provides predetermined add-on signals to image signals.
2. Description of the Related Art
In recent years, the performance of an image forming apparatus, such as a color printer and a color copying machine, has improved, and a high-quality image can be formed. Under the circumstances, images similar to paper currencies, banknotes, and securities can be formed, and problems of forgery of paper currencies and securities as well as copyright infringement may increase in the future.
The following example is known as a control measure for the problems. The technique is a signal add-on system for adding add-on signals which form information related to an image forming apparatus, such as tracking information of a serial number, to a color image formed by the image forming apparatus, the add-on signals being hard to be identified by human eyes (for example, see Japanese Patent Application Laid-Open No. 2002-202650). In the system, when faked paper currencies are discovered, an analysis apparatus that analyzes the tracking information can specify the image forming apparatus that has forged paper currencies based on the add-on signals formed on the paper currencies.
The add-on signals are usually added to the entire image. To add add-on signals to, for example, a color image constituted by yellow, magenta, cyan, and black planes to make the add-on signals hard for the human eyes to identify, the add-on signals are usually added only to the yellow plane having low visibility.
Adding the add-on signals to the color image allows one to extract the add-on signals from the image and restore the tracking information, when an image prohibited from being formed or a copied image prohibited from being copied is discovered. As a result, the image forming apparatus that has formed the image can be specified.
The add-on signals are added to all output images regardless of whether the image formation or copying is prohibited or not prohibited. Meanwhile, the image forming apparatus usually executes halftone processing (halftone processing), and halftone dots are formed by the halftone processing based on input image data.
Under the circumstances, Japanese Patent Application Laid-Open No. 2001-103285 discloses a technology for making an image visually less uncomfortable by off-dotting adjacent pixels upon the addition of an add-on signal, if there is an image adjacent to the add-on position.
However, the positional relationship between the halftone dots and the add-on signals changes according to the relationship between the period of the formed halftone dots and the add-on period of the add-on signals in Japanese Patent Application Laid-Open No. 2002-202650. As a result, the distinction between the halftone dots and the add-on signals is difficult in areas where the halftone dots overlap the add-on signals, causing a problem in which the detection accuracy of the add-on signals is significantly degraded. Meanwhile, there is a problem that the add-on signals are combined with the halftone dots in the areas where the halftone dots and the add-on signals are adjacent to each other, when the size of the add-on signals is made large enough to prevent the foregoing problem, and the add-on signals are easily spotted, resulting in degradation of the image quality.
Japanese Patent Application Laid-Open No. 2001-103285 is significantly useful in that visually less uncomfortable add-on signals can be add-on. However, there is a problem that the processing load for off-dotting the pixels adjacent to the add-on signals is heavy, because the normal image signals and the add-on signals are asynchronous.
Because of these factors, an image forming apparatus that can add add-on signals at appropriate positions with less processing load is desired.